The high speed document counters in use today use several methods of detecting the improper condition of more than one document passing through the feed path at one time, such a feed being called a "double". One of these prior art means involves using a light emitting diode (LED) placed on one side of the document feed path and focused on a light detector on the other side of the document feed path. The amount of light from the LED is kept constant. As a document passes between the LED and the light detector, the amount of light passing through the document is determined by the light detector in terms of current or voltage in the light detector circuit.
One problem with the above-described doubles detection method is that there is non-linearity of the output of the light detector as a function of the amount of light striking the detector over a wide range of brightness. The wide range of brightness is encountered because there is full brightness with no document in the line of detection and there is little brightness when a document such as a U.S. dollar bill is in the line of detection because of the opaqueness of the U.S. dollar bills. Furthermore, there is little difference in the brightness detected between a single U.S. dollar bill and a double U.S. dollar bill as compared to no documents versus a single document present. Therefore, the circuit required to compensate for this nonlinearity must be complex.
Another problem with the above-described prior art doubles detection method is that there is a variation in the construction of the light detectors so that the light detection circuit has to be designed to take this variation into account and a complex setup method must be provided so that all document counters operate the same.
In addition to doubles detection, high speed document counters must also provide a means for detecting if two or more successive documents pass through the counter feed path with no gap between them, this being referred to in the art as a "chain". In order to be able to detect a chain, the counter must be able to "measure" the width of the document after it detects the leading edge so as to determine that the trailing edge comes within an expected range.
The problem faced with chain detection is that the time that a document passes through the counter is variable and depends on where it falls within a batch. The first document in the batch is critical because it is more likely to produce a chain and so must be checked most precisely. The middle documents must be measured with average precision and the last document of the batch is slower than the other documents because the drive motor decellerates as this last document is fed through the counter.
These variations in precision and time are circumvented in the prior art machines by the use of a timing wheel. A timing wheel is a disc with evenly spaced holes around the outer part of the wheel. The wheel is placed on one of the main shafts of the document counter so that its rotational speed is approximately in proportion to the rate at which documents pass through the counter. A sensor-LED combination "sees" the holes in the timing wheel and produces timing pulses. Using these timing pulses, instead of real time, allows the counter to measure each document with enough precision to determine whether a chain of documents passes through the counter. However, the use of a timing wheel is expensive and increases the cost of the counter substantially.